THE CHEMISTRY OF LIFE 37 



involved, however, we still refer to oxygen as an element. This is 

 true of most of the gaseous elements, such as nitrogen (N 2 ) and chlo- 

 rine (Cl 2 ) as well as oxygen. The majority of the liquid and solid 

 elements, however, exist in the pure state without such a combination 

 of their atoms into molecules. 



There are about 100 different kinds of atoms that are known and 

 hence an equal number of elements. However, through combinations 

 of these atoms into various kinds of molecules, literally millions of 

 different kinds of compounds can be formed. About 500,000 compounds 

 have been identified and catalogued, but many more than this number 

 exist or can be produced. Water is one of the simpler compounds. 

 Common table sugar (sucrose) is somewhat more complex — each of its 

 molecules is composed of 12 atoms of carbon, 22 atoms of hydrogen, 

 and 11 atoms of oxygen (C^H^On)- Some of the more complex 

 compounds contain hundreds of atoms in each molecule. 



Mixtures are combinations of different elements or compounds in 

 which there is little if any chemical union of the combined parts. Fur- 

 thermore, mixtures can be composed of different combinations of parts, 

 but compounds always have the same proportions of elements in their 

 make-up. For an example, we can mix water and alcohol and get a 

 homogeneous fluid, but this is not a compound. We can make a water- 

 alcohol mixture of many different proportions. This is not true of 

 compounds — the same amount of carbon, hydrogen, and oxygen must 

 always be present in sucrose. Since mixtures can be composed of dif- 

 ferent proportions of parts, it is evident that the chemical and physical 

 properties of a mixture will vary with the variations in its component 

 parts. A compound, on the other hand, having a fixed proportion of 

 parts, will exhibit fixed chemical and physical properties. 



The air around you is a familiar example of a mixture. The air 

 which you are now inhaling is a mixture of nitrogen (an element), 

 oxygen (an element), carbon dioxide (a compound), and vaporized 

 water (a compound) which will vary in quantity depending upon the 

 humidity of the air. There will also be very small amounts of other 

 gases, perhaps some smoke, and a few other assorted things in this sur- 

 rounding envelope which we call air. When you exhale air its pro- 

 portions of oxygen, carbon dioxide, and water vapor will be different, 

 but it will still be air. 



The Nature of Atoms and Molecules 



Since atoms are the basic building blocks of all states of matter 

 we may well investigate their nature more fully. There was a time in 



